The PAOs have been recognized for over 30 years as a class of materials which are exceptionally useful as high performance synthetic lubricant base stocks. They possess good flow properties at low temperatures, relatively high thermal and oxidative stability, low evaporation losses at high temperatures, high viscosity index, good friction behavior, good hydrolytic stability, and good erosion resistance. PAOs are not toxic and are miscible with mineral oils, other synthetic hydrocarbon liquids, fluids and esters. Consequently, PAOs are suitable for use in engine oils, compressor oils, hydraulic oils, gear oils, greases and functional fluids.
PAOs may be produced by the use of Friedel-Craft catalysts such as aluminum trichloride or boron trifluoride with boron trifluoride being the catalyst of choice. Boron trifluoride is advantageously combined with a protic promoter, typically an alcohol such as isopropanol or butanol, water, or an acid, ester or an ether, to form a catalyst complex which may be used to promote oligomerization into products with the desired molecular weight highly branched oligomeric structure required for a combination of low pour point and high viscosity index in the lubricant products. The alpha olefins which are generally used are those in the C8 to C14 range, advantageously 1-octene, 1-decene and 1-dodecene; the use of odd carbon number olefins has been found to be unfavorable. Olefins of this type may typically be formed by cracking or by the ethylene chain growth process. The boron trifluoride catalyst normally used in the oligomerization represents a significant cost in process schemes for producing polyalphaolefins since it is generally not recovered but, instead, inactivated by a water wash. The used, inactivated catalyst is often disposed of by deep-well injection in commercial operations producing polyalphaolefins, a disposal method which has some environmental considerations although various methods for recovering the boron trifluoride have been proposed.
In current low viscosity PAO process using the Friedel-Craft catalysts, the dimer or light fractions are recycled into the linear alpha-olefin feed to produce more lube base stock. These dimer or light fractions comprising mostly C8H16 to C30H60 oligomers (average C20H40), exhibit a relatively low average molecular weight of 280 or less, and are not very desirable as feed stock for the process because the isomerization which accompanies the oligomerization process, although valuable in terms of producing branched-chain higher oligomers which are excellent lubricants with high viscosity index and low pour point, also results in the dimer or light fraction composed of the lower oligomers which are themselves highly branched, highly substituted products with an unsaturated double bond in the middle of the molecule; they may be generally characterized as oligomers in the stated molecular weight range with significant short chain branching and highly substituted double bonds (tri- and tetra-substituted olefins). Being sterically hindered, the double bonds in these light co-products are less accessible and therefore less amenable to further reaction. Thus, these dimer or light fractions are less reactive toward further oligomerization. Furthermore, they are more highly branched olefins and the lube products from these branched molecules have less desirable VI, volatility and thermal/oxidative stability as a consequence of their structure.
The demand for high quality PAO, especially low viscosity/high VI/low pour point PAO is increasing fast and alternatives to the current Friedel-Craft process are being proposed. Supported, reduced chromium catalysts and metallocene catalyst systems have been proposed for such processes. A process using a metallocene catalyst for the production of 4 to 10 cS, low viscosity PAO base stocks is described in WO 2007/011973 (Wu et al). This technology is attractive because the metallocene-based low viscosity PAO has excellent lube properties. One disadvantage of this process so far, has been that when producing 4-10 cS PAO, some amount of dimer or light oligomers, smaller than C30 are obtained as co-products. These light olefins cannot be used as lubricant base stocks as they are too volatile and they cannot be recycled into the metallocene-catalyzed process because they are usually more linear and with high degree of vinylidene or vinyl contents. The light fractions from the metallocene oligomerization process therefore represent a lube yield loss if they cannot be converted into lube products by other methods.
Another alternative to the Friedel-Craft catalyzed process is the process using an ionic liquid catalyst. Ionic liquids are liquids that contain essentially only ions rather than molecular species; the term is commonly used for salts whose melting point is relatively low (typically below 100° C.). Processes of this type may be used for producing high viscosity PAOs as described, for example, in U.S. Pat. Nos. 7,259,284; 6,395,948 and U.S. Patent Applications Nos. 2006/0247482; 2005/0113621; 2004/0030075 and 2002/0128532 and EP 0 791 643. An alpha-olefin oligomerization process using a non-nickel transition metal catalyst and an ionic liquid medium is described in U.S. Patent Application No. 2002/0183574 and in this case, the product is reported to be a trimer. A two-step process in which an initial oligomerization is carried out using a metallocene catalyst preceding an oligomerization of the lower molecular weight product using a Friedel-Craft catalyst is described in U.S. Pat. No. 6,548,724.